The safe and efficient removal of contaminants from an underlying substrate surface can present a formidable challenge. Frequently the contaminants include materials that if not themselves hazardous, can become hazardous during a decontamination process. In addition, the decontamination process should not weaken or substantially damage the structural integrity of the underlying substrate. The design of a system for safely removing contaminants must concern itself with the potential exposure of removal personnel to hazards, cost of the system, the cost of any protective gear required to be worn by those using the system, the cost of the labor required for the decontamination process, and the cost of disposing of decontamination process byproducts.
As used herein, the term "contaminant" need not designate a material that is intrinsically harmful. For example, a coating such as paint, whose removal from an underlying substrate is desired, may be deemed a contaminant.
One commonly used prior art approach has been to blast the contaminants away with abrasive particles such as sand or plastic beads. While the equipment required to practice this approach is relatively inexpensive, this process is highly labor intensive, requiring protective masking of adjacent substrate regions and the wearing of protective garments by the work crew. During decontamination, considerable grit and/or particulate dust is present. This particulate matter often dictates that adjacent electrical generators and similar equipment be shutdown and protected, the downtime representing an additional economic burden imposed by abrasive decontamination systems.
In many applications, abrasive decontamination processes must be performed within an enclosed housing, which requires that the substrate be brought to the housing. This requirement can be burdensome where, for example, the substrate is large or cumbersome, the hull of a seagoing vessel, for example. Further, abrasive processes are slow, typically being on the order of less than one square foot per minute. Further, the structural integrity of the substrate being decontaminated may be weakened due to dimpling or stretching from impact with the abrasive particles. Finally, after decontamination is complete, a considerable volume of contaminated grit, including for example water, CO.sub.2, plastic media and the like, must be safely disposed of, thus imposing a burden on existing landfill resources.
A second commonly used prior art approach is the use of chemical agents to remove contaminants such as undesired paint, methylene chloride being a commonly used agent. Unfortunately chemical agent techniques are even more labor intensive than abrasive techniques, requiring extensive preparation and clean-up after stripping, requiring perhaps 250 man-hours to decontaminate the exterior of a commercial airliner. Further, the personnel performing the decontamination must be provided with costly and cumbersome protective full body suits and breathing apparatus. Finally, chemical decontamination process byproducts can include hundreds of gallons of contaminated water and often methylene chloride, for which the cost of safe disposal can be quite high. At present there are few options available with regard to a chemical stripping agent that is effective and safe. Methylene chloride, for example, is expected to be banned by the United States Environmental Protection Agency from future use due to its release of ozone-depleting chlorofluorocarbons ("CFCs").
A third prior art approach is the removal of contaminants using high intensity visible spectrum light energy. For example, as disclosed in U.S. Pat. No. 4,867,796 to Asmus, et al., the contaminant is first precoated with an energy absorbing medium, and then subjected to pulses of high intensity light energy. The medium absorbs the light energy, which is converted to heat causing the contaminant to decompose and/or be vaporized, thus removing the contaminant from the substrate. The heat generated by the short duration light energy pulses is localized at the contaminant surface and is safely dissipated by the accompanying contaminant vaporization without substantially affecting the substrate. Interestingly, it has long been held in the prior art that energy pulses exceeding about 20 Joule/cm.sup.2 are undesirable as tending to unduly heat and stress, if not combust, the underlying substrate.
While Asmus-type systems are especially promising commercially, the need to precoat the substrate before decontamination is time consuming, costly, and potentially hazardous. For example, workers performing the decontamination process are exposed to potentially hazardous contaminants during precoating. Even if the contaminant being precoated is non-hazardous, the areas to be precoated may be difficult or dangerous to reach, a very high ceiling, for example. After decontamination the problem remains of how to safely dispose of hazardous contaminants once they have been removed from the substrate using light energy. Finally, the thermal energy associated with Asmus-type decontamination systems can cause even non-hazardous contaminants to breakdown into sub-components that are hazardous and require safe disposal.
To summarize, what is needed is an apparatus and method providing safe and efficient decontamination, without damaging the substrate and without requiring that the contaminant be precoated. Preferably the apparatus and method should result in the removed contaminant being reduced to constituents that are relatively non-hazardous, and should provide a mechanism for containing and removing such constituents from the work site. Further, safe and efficient decontamination should be provided without requiring personnel performing decontamination to wear expensive and cumbersome bodysuits and breathing apparatus. The present invention discloses such an apparatus and method.